Process for preparing tertiary alkyl ethers

ABSTRACT

A process for preparing tertiary alkyl ethers comprises the steps of reacting C 4-7  isoolefins of an olefinic hydrocarbon feedstock with an alkanol in the presence of a first catalyst in a reaction zone to form a reaction mixture containing a tertiary alkyl ether or a mixture of tertiary alkyl ethers, feeding the reaction mixture to a distillation column, distilling the reaction mixture and recovering the alkyl ether(s) with the bottoms product of the distillation. An azeotrope formed by unreacted C 4  hydrocarbons and the alkanol is withdrawn as an overhead product of the distillation. According to the invention, a part of the liquid flow of the column is withdrawn to form a side drawoff, and the side drawoff is recirculated to the reaction zone. As a result, the conversion of the reactive C 6  hydrocarbons is increased and the operating cost of the ethers process is reduced.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/FI96/00677 which has an Internationalfiling date of Dec. 19, 1996 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a process for preparing tertiary alkylether products which are used, in particular, as components of motorfuels. The products contain, methyl t-butyl ether, ethyl t-butyl ether,t-amyl methyl ether or t-amyl ethyl ether, or mixtures thereof, andpossibly heavier tertiary alkyl ethers. According to the process,isoolefins, in particular the C₄ -C₇ isoolefins, of an olefinichydrocarbon feedstock are reacted with a suitable alkanol in order toprepare the corresponding ethers. These ethers are removed together withthe bottoms product of the distillation reaction system and, ifnecessary, they are further processed in order to prepare a motor fuelcomponent. Unreacted alkanol is removed with the overhead product of thedistillation.

2. Description of Related Art

Tertiary alkyl ethers are added to gasoline in order to improve theanti-knocking characteristics thereof and to reduce the concentration ofharmful components in the exhaust gases. The oxygen-containing ethergroup of these compounds has been found favourably to improve thecombustion process of automotive engines. Suitable alkyl tert-alkylethers are methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE),t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methylethers (THME), to mention a few examples. These ethers are prepared byetherification of isoolefins with monovalent aliphatic alcohols(alkanols). The reactions can be carried out in a fixed bed reactor, ina fluidized bed reactor, in a tubular reactor or in a catalyticdistillation column.

The etherification reaction is an exothermic equilibrium reaction, andmaximum conversion is determined by the thermodynamic equilibrium of thereaction system. To use TAME as an example, it is possible to obtain anabout 90% conversion by carrying out reaction and separation in areactive distillation column, whereas only a 65 to 70% conversion isobtainable in a fixed bed reactor.

Ion exchange resins are the most common etherification catalysts.Generally the resin used comprises a sulfonated polystyrene/divinylbenzene based cation exchange resin (sulfonated polystyrene cross-linkedwith divinylbenzene) having particle sizes in the range from 0.1 to 1mm.

Two types of TAME processes have been commercially available for sometime. The first one comprises fixed bed reactors, columns for productseparation by distillation and a methanol separation unit. In the otherprocess, the product distillation is replaced by a catalyticdistillation unit, which substantially improves the TAME conversion, asmentioned above.

A third completely novel etherification process is described in ourInternational Patent Application WO 93/19031. This novel processcomprises a catalytic distillation unit which has been modified bytransferring the catalyst conventionally placed in the distillationcolumn into a separate external reactor which is being fed from theproduct separation distillation unit. The side reactor product isrecycled back to the same product separation distillation unit.According to an embodiment of that process described in ourinternational patent application WO 93/19032, the product distillationof the catalytic distillation reactor system is operated in such a waythat most, and preferably practically all, of the alkanol which isremoved with the distillate is bound to the inert C₄ hydrocarbons of thedistillate, forming an azeotrope with them. The product is recoveredfrom the bottom of the column and it comprises a mixture of TAME andheavier ethers.

The process described in our international patent applications mentionedabove can also be used for preparing lower alkyl ethers, such as methylt-butyl ether (MTBE) and ethyl t-butyl ether (ETBE), and mixed etherproducts containing such ethers.

A suitable feedstock for the above-mentioned processes for preparingtertiary alkyl ethers is Fluidized Catalytic Cracking (FCC) Gasolinecontaining C₄₋₇ hydrocarbons, a substantial portion, generally at least5%, typically about 7 to 30 wt-%, of which comprises reactive C₄₋₇isoolefins. These reactive isoolefins include the following compounds:isobutene, 2-methyl-1-butene, 2-methyl-2-butene, 2-methyl-1-pentene,2-methyl-2-pentene, 2,3-dimethyl-1-butene, 2,3-dimethyl-2-butene,2-ethyl-1-butene, 2-methyl-2-hexene, 2,3-dimethyl-1-pentene,2,3-dimethyl-2-pentene, 2,4dimethyl-1-pentene, 2-ethyl-1-pentene and2-ethyl-2-pentene. Other suitable hydrocarbon feedstocks foretherification processes are formed by Pyrolysis C₅ Gasoline, ThermoforCatalytic Cracking (TCC) Gasoline, Residual Catalytic Cracking (RCC)Gasoline and Coker Gasoline.

Although the above-mentioned novel etherification process will provideexcellent conversion rates of the reactive C₄ 's and C₅ 's, theconversion of the reactive C₆ 's to the corresponding tertiary alkylethers (e.g., THME, tert-hexyl methyl ether, THEE, tert-hexyl ethylether) is less than 50%. Depending on the process configuration it caneven be less than 40 or 30%. In a mixture containing C₄, C₅ and C₆-based ethers and the corresponding non-reactive hydrocarbons, anincrease of the amount of C₆ ethers would significantly reduce the vaporpressure of the ether products, improve the octane number thereof and,considering the fact that the alkanol is a more inexpensive componentthan the gasoline, increase the cost efficiency of the process.

SUMMARY OF THE INVENTION

The present invention aims at eliminating the problems associated withthe prior art by providing a novel process for producing tertiary alkylethers from an olefinic hydrocarbon feedstock while increasing theconversion of reactive C₆ hydrocarbons to over 50% and maintaining highconversion rates (over 90%) of reactive C₄ and C₅ hydrocarbons.

The present invention is primarily based upon the novel etherificationprocess described in international patent application WO 93/19032. Inparticular, the hydrocarbon feedstock and at least one alkanol are fedinto a reaction zone, wherein the components of the feed, viz., thealkanol(s) and the reactive isoolefins, are reacted with each other inorder to form a product containing tertiary alkyl ethers. The reactionmixture is continuously subjected to fractionation in a distillationcolumn. A bottoms product mainly containing the alkyl ethers formed andsubstantially all of the unreacted hydrocarbons is withdrawn from thedistillation, whereas the overhead product mainly contains an azeotropeformed by the non-reactive (inert) feed hydrocarbons, in particular thenon-reactive C₄ hydrocarbons, and alkanol not consumed by theetherification reaction.

According to the present invention the feed stream containing thehydrocarbons together with the alcohol is combined with a recycle streamfrom the fractionator (distillation column) before it is being fed intothe etherification reaction zone. Surprisingly, it has turned out thatby recycling from the distillation column a drawoff at a rate of 10 to500%, preferably about 50 to 200%, of the fresh feed stream andcombining it with the fresh feed, it becomes possible to increase theconversion of the reactive C₆ 's to over 65% compared with less than 50%if the side drawoff is subjected to etherification separately.

In particular the present invention comprises the following steps:

reacting C₄₋₇ isoolefins of an olefinic hydrocarbon feedstock with analkanol in the presence of a first catalyst in at least one reactionzone to form a reaction mixture containing a tertiary alkyl ether or amixture of tertiary alkyl ethers,

feeding the reaction mixture to a distillation column at a feed pointbetween the bottom and the top of the column,

subjecting said reaction mixture to distillation in the distillationcolumn,

recovering the alkyl ethers and C₅₋₇ hydrocarbons with the bottomsproduct of the distillation,

withdrawing as an overhead product of the distillation an azeotropeformed by unreacted C₄ hydrocarbons and said alkanol,

withdrawing a part of the liquid flow of the column from above the feedpoint of the reaction mixture to form a side drawoff, and

recirculating the side drawoff to the reaction zone.

BRIEF DESCRIPTION OF THE DRAWING

Next, the invention will be described in more detail with the aid of theattached drawing, which depicts a simplified scheme of an etherificationprocess according to the present invention, comprising three prereactorsand a product separation column.

DETAILED DESCRIPTION OF THE INVENTION

The production of the ether according to the present invention can becarried out in a conventional etherification system comprising a numberof reactors in a cascade connected to at least one distillation columndesignated for product separation. Typically, in such a processconfiguration, the feed hydrocarbons together with the alcohol (methanolor ethanol) and the recycle stream from the fractionator are fed to thereaction zone, which comprises at least two reactors. The greater theratio of heavier hydrocarbons to light hydrocarbons, the more reactorsare needed. The feed is first adjusted to the specific reactiontemperature used before feeding to first etherification reactor. Theeffluent from the first reactor is cooled and fed to a secondetherification reactor. The effluent from the second reactor isoptionally cooled and fed to a third (fourth etc.) etherificationreactor. The effluent of the last reactor is then heated and fed to themain fractionator, which is operated according to the principles laiddown in WO 93/19032, i.e. so that the distillate consists of mainly C₄hydrocarbons and the alcohol, which is in azeotropic concentration inthe distillate. The amount of unreactive feed C₄ 's therefore fixes theamount of distillate. A side drawoff is taken out from the distillationcolumn above the feed point and fed to the first reactor via a heatexchanger. The bottom product consists of unreacted hydrocarbons and theethers formed.

In the above embodiment, the fresh feed, the alkanol and the sidedrawoff are mixed together before feeding into the etherification zone.It is also possible to feed one (e.g. the alkanol) or all of the threestreams separately into the etherification zone. In that case, the sidedrawoff is preferably cooled before being fed into the reaction zone.

The term "alkanol" includes lower alkyl alcohols capable of formingazeotropes with saturated and unsaturated hydrocarbons, in particular C₃to C₇ hydrocarbons, of the hydrocarbon feedstock. As specific examplesof the alkanols, the following can be mentioned: methanol, ethanol,n-propanol, i-propanol, n-butanol, i-butanol and t-butanol. Methanol andethanol are particularly preferred.

The term "olefinic hydrocarbon feedstock", is intended to cover allhydrocarbon feedstocks, which contain an isoolefin or a mixture ofisoolefins which can be etherified to form tertiary alkyl ethers. Inparticular, the following feedstocks are preferred: FCC Gasoline, FCCLight Gasoline, Pyrolysis C₅ Gasoline, TCC Gasoline, RCC and CokerGasoline. The feed can also comprise a mixture of two or more olefinichydrocarbon feedstocks, such as a mixture of FCC Light Gasoline and apyrolysis C₅ cut. The proportion of the various C₄ to C₇ isoolefinswill, of course, to a large extent determine the composition of theether product.

Of the above feedstocks, FCC, RCC and TCC are preferred because thesehydrocarbon cuts can be used as such, possibly after the removal ofheavier cuts (C₈₊). The use of Pyrolysis Gasoline requires that thelight cut and the C₆₊ cut be removed before it can be fed into theprocess. Up to some 10% of the C₆₊ cut can be included in the resultinghydrocarbon mixture, called a Pyrolysis C₅ Gasoline, so as to ensurethat substantially all of the reactive C₅ 's of the Pyrolysis Gasolineare present in the olefinic feedstock. This feedstock will also containreactive aliphatic C₆₊ hydrocarbons. Pyrolysis Gasoline is particularlyrich in isoprene (up to 10 wt-%) and other diolefins, which can beconverted to mono-unsaturated hydrocarbons by selective hydrogenation.This will greatly improve the value of this cut as a feedstock foretherification, in particular in combination with any of the abovementioned cracking gasoline cuts.

The attached drawing gives an overview of a preferred embodiment of theprocess according to the present invention.

Thus, in the test arrangement depicted in the drawing, the hydrocarbonfeedstock, the alkanol, and a side stream from distillation column 4 aremixed together, the mixture is heated and fed through the reactorsection 1, 2. The hydrocarbon feedstock may, for instance, be ahydrocarbon fraction containing isoolefins, such as a hydrocarbon cut ofa cat cracker, containing a mixture of isoolefins. The reactors consistof three reactors filled with ion exchange resin beds. The reactors canbe fixed or fluidized bed or tubular reactors. The reactors may bearranged in series (in a cascade), as shown in the figure, or inparallel. If there are more than two reactors they may also be arrangedin series/parallel. Because of the reaction there is a temperature risein the prereactors in the range from about 5 to about 15° C. dependingon the amount of isoolefins and the efficiency of the reactorinsulation. From the reactors the mixture is conducted to distillationcolumn 4. The location of the feed point is defined below morespecifically. At the bottom of the distillation column 4 there is areboiler 9. The distillation column can be a packed column or oneprovided with valve, sieve or bubble-cap trays. The overhead of thecolumn is removed via a condenser 10 to a reflux drum 11, from which theoverhead is removed by means of a pump 12. A part of the overhead isforwarded to further processing and a part thereof is returned to thedistillation column. Ethers are removed with the bottoms product. Inaddition to the ether, the bottoms product also contains unreacted C₄₊hydrocarbons. The reflux ratio of the column is preferably from about 1to 500. Even greater ratios can be used in pilot plant equipments.According to the invention, the reflux ratio is adjusted so that thedistillate amount removed from the process at least substantiallycorresponds to the amount of C₄ hydrocarbons of the feed.

From the distillation column 4 a side stream is taken and mixed withfresh hydrocarbon and alkanol feeds as described above. The side drawoffcomprises some 10 to 500%, preferably about 50 to about 200% of thefresh feed. The pressure of the side stream is increased by pump 14because the distillation is typically carried out at a lower pressurethan the reaction. The side stream is preferably taken from a tray whichis located below trays having alkanol K-values less than 1. The effluentof the reactors (distillation column feed) is fed to a plate having analkanol K-value greater than 1. As a result of this arrangement, thealkanol gets more enriched in the vapor phase than do the hydrocarbons.The side drawoff makes up 40 to 90%, typically about from 60 to about70% of the total liquid flow within the column.

The distillation is carried out at a pressure generally ranging fromabout 1.1 to 20 bars and the etherification reaction at 6 to 40 bars.When preparing TAME, the temperature at the top of the distillationcolumn is about 40 to 70° C., typically about 50 to 60° C., and at thebottom of the column about 100 to 150, typically about 120 to 130° C.

As mentioned above, according to the present invention the distillationcolumn of the reactive distillation unit is operated in such a way thatthe alkanol is heavier than the hydrocarbons at the top of thedistillation column. Therefore, the alkanol not bound to thehydrocarbons in the form of an azeotrope will tend to flow downwardswithin the column. At the same time the vapor-liquid-equilibrium betweenC₅ and heavier hydrocarbons and the alkanol at the bottom of the columnis maintained at such a level that the alkanol is lighter than thehydrocarbons. This causes the alkanol to flow upwards from the bottom ofthe column. Thus, the alkanol will circulate within the distillationsystem between the top and the bottom of the column. By fitting areaction bed in the distillation column or by conducting a side streamfrom the column through a reaction bed in a side reactor, an alkanolconsuming reaction can be created which will remove the alkanol from thesystem.

The alkanols, in particular methanol and ethanol, form azeotropes withthe hydrocarbons of the feedstock. The heavier the hydrocarbons, thegreater the alkanol concentration of the hydrocarbon-alkanol-azeotrope.According to the present invention, in order to minimize the amount ofunreacted alkanol removed from the distillation process, substantiallyonly the C₄ -hydrocarbon-alkanol azeotropes are taken as an overheadproduct. These azeotropes are the lightest hydrocarbon-alkanolazeotropes and have the smallest alkanol concentrations.

Thus, according to the present invention, the amount of unreactedalkanol can be controlled by adjusting the amount of C₄ hydrocarbons inthe feed so that it correlates with the amount of alkanol. The lessthere are C₄ hydrocarbons in the feed, the less distillate can beremoved and the less alkanol is removed from the process. By increasingthe amount of C₄ hydrocarbons in the feed the distillate flow rate canbe increased without any change of the relative amount of free unreactedalkanol in the overhead product. Therefore, if desired, C₄ hydrocarbons(or even C₃ hydrocarbons) can deliberately be added to the process sothat the intended effect is achieved.

When operating the process according to the invention, the alkanolconcentration of the bottoms product of the column can easily be reducedto as small a value as desired. In the case of methanol, it is possibleto reduce its concentration in the bottoms product to below 100 ppm. Theamount of alkanol in the distillate will correspond to the amount boundby the azeotrope, only. The composition of the azeotrope and, thus, theamount of alkanol removed depends on the hydrocarbon composition of theoverhead product and the operating pressure of the distillation. Tomention an example based on the production of TAME: if C₄ hydrocarbonsmake up the main part (over 90%) of the overhead product, there willremain some 0.1 to 5.0% by weight of methanol depending on thedistillation pressure and the amount of C₅ hydrocarbons. The more C₅hydrocarbons are included in the overhead product, the more methanolwill be removed with it (there may be less than 90% by weight of the C₄hydrocarbons in the overhead product).

The above-described etherification is preferably carried out in thepresence of a conventional cation exchange resin. However also differentkinds of zeolites can also be used as etherification catalysts. Thus,the resin may contain sulfonic acid groups and it can be obtained bypolymerization or copolymerization of aromatic vinyl compounds followedby sulfonation. Examples of aromatic vinyl compounds suitable forpreparing polymers of copolymers are: styrene, vinyl toluene, vinylnaphthalene, vinyl ethyl-benzene, methyl styrene, vinyl chlorobenzeneand vinyl xylene. The acid cation exchange resin typically contain some1.3 to 1.9 sulfonic acid groups per aromatic nucleus. Preferred resinsare based on copolymers of aromatic monovinyl compounds with aromaticpolyvinyl compounds, particularly, divinyl compounds, in which thepolyvinyl benzene content is from about 1 to 20 wt-% of the copolymer.The ion exchange resin preferably has a granular size of about 0.15 to 1mm.

In addition to the above resins, perfluorosulfonic acid resins which arecopolymers of sulfonyl fluorovinyl ethyl and fluorocarbon can be used.

The invention is preferably carried out in connection with the MTBE,ETBE, TAME and the TAEE processes.

In connection with the TAME process, the overhead product obtained canbe forwarded to a MTBE unit. Since it contains some impurities (C₅hydrocarbons, as far as the MTBE process is concerned), the overheadproduct can be introduced either in the feed of the MTBE unit, whichmeans that the C₅ hydrocarbons remain in the MTBE product, or to themethanol washing unit of the MTBE unit. In the latter case the C₅hydrocarbons end up in the raffinate stream of the MTBE unit (whichcontains mainly C₄ hydrocarbons).

Alternatively, the overhead product of the distillation can--because itcontains only minute amounts of methanol and because the overhead isvery small compared to the feed--also be combined with the bottomsproduct of the distillation in order to form a gasoline component. Ifnecessary, the mixture is subjected to an additional treatment.According to a preferred embodiment of the invention, the C₄ hydrocarboncontent of the feed is, however, deliberately kept so small that themixture of the overhead and the bottoms products can be used as such asa component of motor fuels.

Considerable benefits are achieved by means of the invention. Thus, notonly is the conversion rate of the reactive C₆ 's greatly increased, thepresent invention also reduces the capital investment of theetherification process by simplifying the equipment, and increases theyield of produced ethers compared to the state of art. The operatingcosts are also lowered by reduced utility (steam, water) usage.

The following working example will clarify the invention:

EXAMPLE Preparation of Tertiary Methyl and Ethyl Ethers

Using the process configuration of FIG. 1, methyl and ethyl ethers wereprepared from an olefinic hydrocarbon feed as follows:

Distillation

column: Inner diameter 160 mm, height 11,000 mm, filled with columnpacking. The number of packing layers was 6.

Reactors: Inner diameter 154.1 mm, height 1,150 mm. Filled with thecatalyst DOWEX M-32

Location of

side drawoff: Between the second and third packing layer.

Feed point: Between the fourth and fifth packing layer.

For the preparation of the methyl ethers, two reactors in a cascade wereused, whereas the ethyl ethers were prepared using three reactors in acascade.

In both cases an olefinc feed stream containing 30 kg hydrocarbons/h(compositions shown in Tables 1 and 2 and an alcohol (amounts shown inTables 1 and 2) were mixed together and heated. Then, a side drawoffstream was combined therewith and the modified feed stream thus obtainedwas conducted through the reactors. As a result of the exothermaletherification reaction, the temperature increased in the reactors with5 to 15° C., depending on the efficiency of the heat insulation. Thereaction mixture obtained was conducted to a distillation column andsubjected to distillation.

When preparing methyl ethers, the feed temperature of the reactors was39° C. and temperatures of the reaction mixture streams at the outlet ofthe reactors were 46.5 and 40° C., respectively. In case of ethanol, thefeed streams were fed into the reactors at 59, 59 and 49° C.,respectively, whereas the temperatures of the reaction mixture streamsat the outlets of the reactors were 69, 61 and 50.5° C., respectively.

Distillate was recovered from the top of the distillation column(composition shown in Tables 1 and 2).

The composition of the side drawoff withdrawn from the column is alsoindicated in Tables 1 and 2. The pressure of the side drawoff streamwere increased with a pump before they were conducted to the reactors.

The reflux ratios of the distillation were, in case of MeOH feed, 100and, in case of EtOH feed, 20.

The results obtained are shown in Tables 1 and

                                      TABLE 1                                     __________________________________________________________________________              1         2          10        11        12                           Feed MeOH feed Side draw Top Product Bottom product                                   Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                           Mass-%                                                                             Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                            Mass-%              __________________________________________________________________________    H2O       0.00  0.00                                                                              0.00  0.05 0.00  0.00                                                                              0.00  0.00                                                                              0.00   0.00                  DME 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00                         trans-2-butene 0.63 2.11 0.00 0.00 2.06 4.29 0.63 88.16 0.00 0.00                                                                      3-methyl-1-bute                                                              ne 0.04 0.14                                                                  0.00 0.00 1.44                                                                2.99 0.01 1.47                                                                0.03 0.10                                                                      Iso-pentane                                                                  5.20 17.33 0.00                                                               0.00 28.62                                                                    59.62 0.04 5.96                                                               5.16 16.18                                                                     2-methyl-1-bute                                                              ne 0.46 1.52                                                                  0.00 0.00 0.05                                                                0.10 0.00 0.00                                                                0.01 0.04                                                                      n-pentane 4.22                                                               14.08 0.00 0.00                                                               6.88 14.33 0.00                                                               0.13 4.22 13.26       2-methyl-2-butene 3.27 10.89 0.00 0.00 0.45 0.94 0.00 0.01 0.34 1.06                                                                   n-pentene 0.65                                                               2.18 0.00 0.00                                                                1.03 2.15 0.00                                                                0.01 0.65 2.05                                                                 2,3-dimethyl-bu                                                              tene 3.04 10.12                                                               0.00 0.00 0.50                                                                1.05 0.00 0.00                                                                3.04 9.53                                                                      2-methyl-1-pent                                                              ene 0.66 2.21                                                                 0.00 0.00 0.05                                                                0.10 0.00 0.00                                                                0.38 1.18                                                                      3-methyl-penten                                                              e 1.79 5.96                                                                   0.00 0.00 0.16                                                                0.33 0.00 0.00                                                                1.79 5.61                                                                      MEOH 0.00 0.00                                                               2.58 99.95 6.62                                                               13.78 0.03 4.26                                                               0.00 0.00                                                                      2-methyl-2-pent                                                              ene 3.37 11.23                                                                0.00 0.00 0.06                                                                0.12 0.00 0.00                                                                1.00 3.13                                                                      N-hexane 2.34                                                                7.80 0.00 0.00                                                                0.07 0.15 0.00                                                                0.00 2.34 7.35                                                                 benzene 0.46                                                                 1.53 0.00 0.00                                                                0.02 0.03 0.00                                                                0.00 0.46 1.44                                                                 TAME 0.00 0.00                                                               0.00 0.00 0.01                                                                0.02 0.00 0.00                                                                4.91 15.41                                                                     2-methyl-heksan                                                              e 1.80 6.02                                                                   0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                1.80 5.66                                                                      N-heptane 2.06                                                               6.88 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 2.06 6.47                                                                 TAOH 0.00 0.00                                                               0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.01 0.02                                                                      THME 0.00 0.00                                                               0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                3.67 11.52                                                                     4-methyl-hexene                                                               0.00 0.00 0.00                                                               .00 0.00 0.00                                                                 0.00 0.00 0.00                                                                0.00                  Total Flow, kg/hr 30.00 100.00 2.58 100.00 48.01 100.00 0.72 100.00                                                                   31.87 100.00        Pressure, kPa                                                                           2500      2500       2500      400       438                          Temperature, ° C. 45.0 35.0 68.3 46.7 107.4                          __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________               1         2         10        11        12                           Feed EtOH feed Side draw Top Product Bottom product                                    Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                           Mass-%                                                                            Mass Flow                                                                            Mass-%              __________________________________________________________________________    EtOH       0.00  0.00                                                                              3.56  100.00                                                                            3.74  7.30                                                                              0.02  0.24                                                                              0.01   0.03                  Isobutene 1.70 5.67 0.00 0.00 0.02 0.04 0.06 0.88 0.00 0.00                   2-Methyl-1-butene 1.08 3.60 0.00 0.00 0.13 0.26 0.00 0.00 0.03 0.10                                                                    2-Methyl-2-bute                                                              ne 1.94 6.48                                                                  0.00 0.00 1.52                                                                2.96 0.00 0.00                                                                0.60 2.27                                                                      2-Methyl-1-pent                                                              ene 0.90 3.00                                                                 0.00 0.00 0.02                                                                0.04 0.00 0.00                                                                0.12 0.45                                                                      2-Methyl-2-pent                                                              ene 0.93 3.09                                                                 0.00 0.00 0.03                                                                0.07 0.00 0.00                                                                0.78 2.96                                                                      2,3-Dimethyl-1-                                                              butene 0.15                                                                   0.50 0.00 0.00                                                                0.01 0.02 0.00                                                                0.00 0.02 0.09                                                                 2,3-Dimethyl-2-                                                              butene 0.08                                                                   0.28 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 0.11 0.43                                                                 2-Ethyl-1-buten                                                              e 0.03 0.10                                                                   0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 0.01                                                                      3-Methyl-cis-2-                                                              pentene 0.10                                                                  0.35 0.00 0.00                                                                0.00 0.01 0.00                                                                0.00 0.09 0.34                                                                 3-Methyl-trans-                                                              pentene 0.24                                                                  0.81 0.00 0.00                                                                0.00 0.01 0.00                                                                0.00 0.20 0.75                                                                 1-Methylcyclope                                                              ntene 0.15 0.51                                                               0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.13 0.48                                                                      ETBE 0.00 0.00                                                               0.00 0.00 0.02                                                                0.04 0.00 0.00                                                                2.99 11.31                                                                     TAEE 0.00 0.00                                                               0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                3.97 15.03                                                                     THEE1 0.00                                                                   0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 1.44 5.44                                                                 THEE2 0.00                                                                   0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 0.15 0.55                                                                 THEE3 0.00                                                                   0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 0.14 0.51                                                                 Methyl-syclopen                                                              tylether 0.00                                                                 0.00 0.00 0.00                                                                0.00 0.00 0.00                                                                0.00 0.04 0.15                                                                 i-Butane 6.87                                                                22.91 0.00 0.00                                                               1.90 3.71 6.87                                                                96.07 0.00 0.01       n-Hexane 7.14 23.81 0.00 0.00 0.30 0.58 0.00 0.00 7.15 27.07                  Benzene 0.22 0.72 0.00 0.00 0.00 0.01 0.00 0.00 0.22 0.82                     i-Pentane 7.80 25.99 0.00 0.00 43.34 84.53 0.20 2.81 7.58 28.69                                                                        i-hexane 0.65                                                                2.18 0.00 0.00                                                                0.21 0.42 0.00                                                                0.00 0.66 2.49                                                                 Total Flow,                                                                  kg/h 30.00                                                                    100.00 3.56                                                                   100.00 51.27                                                                  100.00 7.15                                                                   100.00 26.41                                                                  100.00              Pressure, kPa                                                                            1500      1500      1500      400       400                          Temperature, ° C. 25.0 25.0 65.9 27.0 95.8                           __________________________________________________________________________

A comparison of the conversions of the various reactants with thecorresponding results obtained by the TAME process described in WO93/19031 indicates that the conversions of 2-Me-1-butene and2-Me-2-butene are on the same level whereas the conversion of reactingC₆ 's is clearly improved. The results are summarized in the followingtable:

                  TABLE 3                                                         ______________________________________                                        Conversion Comparison                                                                   Feed     Bottom   Distillate                                                                            Conversion                                ______________________________________                                        Present invention (methyl ethers)                                               2-Me-1-butene                                                                             1.52     0.04   0.00    0.98                                      2-Me-2-butene 10.89 1.06 0.01 0.90                                            C.sub.6  reacting 13.44 4.31 0.00 0.66                                      Example 4 of WO 93/19031                                                        2-Me-1-butene                                                                             1.82     0.07   0.03    0.96                                      2-Me-2-butene 11.67 1.32 0.06 0.88                                            C.sub.6  reacting 11.75 5.74  0.48                                          Example 1 of WO 93/19031                                                        2-Me-1-butene                                                                             7.50     0.34           0.95                                      2-Me-2-butene 13.74 6.51  0.50                                                C.sub.6  reacting 6.91 4.94  0.24                                           ______________________________________                                    

I claim:
 1. A process for preparing tertiary alkyl ethers, comprisingthe steps ofreacting C₄₋₇ isoolefins of an olefinic hydrocarbonfeedstock with an alkanol in the presence of a catalyst in at least onereaction zone to form a reaction mixture containing a tertiary alkylether or a mixture of tertiary alkyl ethers, feeding the reactionmixture to a distillation column at a feed point between the bottom andthe top of the column, subjecting said reaction mixture to distillationin the distillation column, recovering the alkyl ether(s) and C₅₋₇hydrocarbons with the bottoms product of the distillation, withdrawingan azeotrope formed by unreacted C₄ hydrocarbons and said alkanol as anoverhead product of the distillation, withdrawing from above the feedpoint of the reaction mixture a part of the liquid flow of the column toform a side drawoff, and recirculating the side drawoff to the reactionzone.
 2. The process according to claim 1, wherein the reaction zone isplaced in at least two reactors connected to the distillation column. 3.The process according to claim 1 or 2, wherein the side drawoff iscooled before being fed into the reaction zone.
 4. The process accordingto claim 3, wherein the alkanol used in the etherification zone isseparately fed into the reaction zone.
 5. The process according to claim1, wherein the fresh hydrocarbon feed stream and the side drawoff of thedistillation are combined before the reaction zone to form a modifiedfeed for the reaction zone.
 6. The process according to claim 1, whereinthe alkanol is methanol or ethanol or a mixture thereof.
 7. The processaccording to claim 1, wherein the reaction mixture is heated beforefeeding into the distillation column.
 8. The process according to claim1, wherein the flow rate of the side drawoff amount is 10 to 500%, ofthe flow of the fresh hydrocarbon feed.
 9. The process according toclaim 8, wherein said flow rate is 50 to 200%.
 10. The process accordingto claim 1, wherein said side drawoff is taken from a tray in saidcolumn which is located below trays in said column having alkanolk-values of less than
 1. 11. The process according to claim 9 or 10,wherein said feed point for said reaction mixture is at a plate in saidcolumn having an alkanol k-value of greater than 1.